I need a tad bit of help. The triggers are fairly simple.
If
player commands
at least 1 zerg extractorCenter location labelled
p1 extract on
zerg extractor at
anywhere.
Wait
100 ms.
Remove
zerg extractor for
player at
p1 extract.
Wait
1 ms.
Create
1 Protoss assimilator at
p1 extract for
player.
Wait
1 ms.
Disable doodad state for
Protoss assimilator at
p1 extract.
Except, it CAN'T create the assimilator because the zerg creep remains from the extractor are left behind.

Try to wait for 3000 miliseconds?
You can't quickly get rid of creep, You will have to just wait for it to go away.
That is the problem with creep, it really slows you up in trying to do anything, it should go after 3000 miliseconds, so just wait long if you can.
After 3000 milliseconds, creep from a hatchery is there. I guess I'll just forget it.